1. Field of the Invention
The present invention relates to electrostatically-actuated capacitive micro electro mechanical system (MEMS) switches, and more particularly, to a horizontally electrostatically actuated capacitive MEMS switch having a low insertion loss, a low pull-in voltage and low power consumption.
2. Description of the Related Art
MEMS switches generally include thermo-electrically actuated switches and electrostatically-actuated switches. The electrostatically-actuated switches are classified into resistive switches and capacitive switches according to a switch operation method.
Researches into individual component parts of a communication system, such as the arrangement of switches, filters, inductors, voltage actuated variable capacitors and antennas that can be connected with low noise amplifiers (LNAs), mixers and oscillators that adopt the MEMS technique, have been conducted.
MEMS switches used for a microwave or a radio frequency (RF) band are very profitable to reduce the size and weight of communications systems, because of their advantages such as a low insertion loss and a nearly-negligible quantity of power consumption compared to existing semiconductor switches. Conventional microwave MEMS switches adopting the infinitesimal processing technique cannot be widely used because of drawbacks such as adhesion-related problems and a high pull-in voltage.
As for these conventional MEMS switches, MEMS switches disclosed in U.S. Pat. Nos. 5,578,976 and 5,880,921, xe2x80x9cMicromechanical Electrostatic K-Band Switchesxe2x80x9d, IEEE MTT-S, 1998, xe2x80x9cMicromechnical Membrane Switches for Microwave Applicationsxe2x80x9d, IEEE MTT-S, 1995, xe2x80x9cMicrowave and mm-Wave MEMS Switchesxe2x80x9d, MEMS PI Meeting at Berkeley, Calif., 1997, and xe2x80x9cPerformance of Low-Loss RF MEMS Capacitive Switchesxe2x80x9d, IEEE Microwave and Guided Wave Letters, Vol. 8, No. 8, 1998 are designed so that electrical signals are switched by a driving method.
Conventional MEMS switches have a structure in which a coplanar waveguide (CPW) is formed on a substrate, and a thin bridge, which is a switching structure, is formed to a predetermined height on the formed CPW.
In the switch shown in FIG. 1, a thin switching structure is supported by a serpentine spring, and on/off switching is achieved by actuation in the direction vertical to the substrate. The switch shown in FIG. 2 has a structure in which a switch structure is supported by a cantilever spring to a predetermined height over the substrate.
This vertically driven MEMS switch has a simple structure, and is easily manufactured. However, in this vertically driven MEMS switch, the weight of a switch structure capable of being supported by a spring is restricted. In particular, upon the manufacture of this MEMS switch, the switching structure sticks to the substrate.
A method of manufacturing a vertically driven MEMS switch is conceptually shown in FIGS. 3A and 3B. Referring to FIGS. 3A and 3B, a switching structure 2 is formed on a sacrificial layer 3 formed on a substrate 1 to a predetermined thickness. Then, the sacrificial layer 3 is removed to fly the switching structure over the substrate 1 to a predetermined height.
The process of removing the sacrificial layer 2 is accompanied by a rinsing process using dionized water. At this time, as shown in FIG. 4A, dionized water 4 that generates a strong capillary attraction between the substrate 1 and the switching structure 2 exists therebetween. After drying, as shown in FIG. 4B, the switching structure 2 sticks to the substrate 1.
The adhesion of the movable switching structure to the substrate is achieved not only by the capillary attraction generated by dionized water but also by various factors such as solid bridging caused by the covalent bond of nonvolatile impurities produced by rinsing and drying, an electrostatic force generated by a remanent charge moved to the surface of a switching element after having been accumulated around a switching structure, and a van der Waals force between molecules existing on the surface of a switching element and a substrate. The adhesion caused by various factors as described above provokes malfunction or degradation of switches, so that it must be absolutely avoided. In order to prevent adhesion of a switching structure, the prior art has tried various methods of increasing the spring constant of a spring structure for supporting a switching structure, of employing rinsing water having a low surface tension, of drying a spring structure using a sublimation method after rinsing the spring structure, of installing a restoration electrode on a spring structure to separate the spring structure from a substrate, and of greatly increasing the gap between a substrate and a spring structure.
To solve the above problem, an objective of the present invention is to provide an electrostatically and laterally actuated capacitive MEMS switch designed to effectively prevent adhesion of a switching structure.
Another objective of the present invention is to provide an electrostatically and laterally actuated capacitive MEMS switch which requires a low pull-in voltage is and has a low insertion loss.
Still another objective of the present invention is to provide an electrostatically and laterally actuated capacitive MEMS switch designed so that linear switching is accomplished by a spring structure.
To achieve the above objectives, the present invention provides an electrostatically actuated capacitive MEMS switch according to a first aspect, including: a substrate; first and second posts installed a predetermined distance apart on the substrate; a first cantilever spring having a rear portion connected to the first post and a leading portion located at the center of the substrate; and a second cantilever spring having a rear portion connected to the second post and a leading portion spaced a predetermined distance apart from the leading portion of the first spring, wherein an insulating layer is formed on the leading portion of at least one of the first and second springs to form a variable capacitor between the leading portion of the first spring and that of the second spring.
In the switch according to the first aspect of the present invention, preferably, a signal input unit and a signal output portion are installed adjacent to the first and second posts, respectively, to be electrically connected to the first and second posts, respectively, and a signal input capacitor and a signal output capacitor are installed between the signal input unit and the first post and between the signal output unit and the second post, respectively, to block the flow of direct current.
Also, preferably, first and second power supply input units are installed adjacent to the first and second posts, respectively, to be electrically connected to the first and second posts, respectively, and first and second inductors with predetermined inductance for blocking electrical signals with a predetermined frequency and greater are installed between the first post and the first power supply input unit and between the second post and the second power supply input unit, respectively.
To achieve the above objectives, the present invention also provides an electrostatically actuated capacitive MEMS switch according to a second aspect, including: a substrate; a first post installed at one side on the substrate; second and third posts installed a predetermined distance apart at the other side on the substrate; a first cantilever spring having a rear portion connected to the first post and a leading portion extending toward the center of the substrate; a second cantilever spring having a rear portion connected to the second post and a leading portion spaced a predetermined distance apart from one side of the leading portion of the first spring; and a third cantilever spring having a rear portion connected to the third post and a leading portion spaced a predetermined distance apart from the other side of the leading portion of the first spring, wherein an insulating layer is formed on at least one side of both sides of the leading portion of the first spring, the inner side of the second spring facing on side of the leading portion of the first spring, and the inner side of the third spring facing on the other side of the leading portion of the first spring, so that a variable capacitor is formed between the leading portions of adjacent springs.
In the electrostatically actuated capacitive MEMS switch according to the second aspect of the present invention, preferably, a signal input unit is installed adjacent to the first post to be electrically connected to the first post, a signal input capacitor is installed between the signal input unit and the first post to block the flow of direct current, first and second selective signal output units are installed adjacent to the second and third posts to be electrically connected to the second and third posts, respectively, and first and second output capacitors are installed between the second post and the first selective signal output unit and between the third post and the second selective signal output unit, respectively, to block the flow of direct current.
Also, a first power supply input unit is installed adjacent to the first post to be electrically connected to the first post, a first inductor for blocking electrical signals with a predetermined frequency and greater by having predetermined inductance is installed between the first post and the first power supply input unit, second and third selective power supply input units are installed adjacent to the second and third posts, respectively, and second and third inductors for blocking electrical signals with a predetermined frequency and greater by having predetermined inductance are installed between the second post and the first selective power supply input unit and between the third post and the second selective power supply input unit, respectively.
In the electrostatically actuated capacitive MEMS switches according to the first and second aspects of the present invention, preferably, the springs are Z-shaped and stepped, each having both lateral portions located on different planes and a slanted middle portion for connecting the two lateral portions. Also, it is preferable that an opening is formed at the center of the substrate, and the springs are located over the opening.